Digital signal processing is becoming more and more important in telecommunication apparatuses. This applies especially to wireless telecommunication. A signal is processed digitally in a transmitter, converted to an analog form and transmitted wirelessly to a receiver, which converts the signal back to a digital form.
In recent years, several different wireless telecommunications standards have emerged. These standards support systems based on GSM, CDMA and WCDMA techniques, for example. From the user's point of view, a transceiver supporting several standards provides most versatile usability. Therefore, transceivers capable of communication in systems based on different technologies have been developed.
In implementing a radio receiver supporting several radio standards, analog base band filtering and A/D-conversion bandwidths have to be carefully adjusted for each standard. This consumes silicon area and increases design complexity and design time. The more signal processing is performed in digital domain, the more efficient the design and operation of a receiver will be. It would be advantageous if the same analog front-end could be used for example for GSM, CDMA and WCDMA transmission with only minor, if any, modifications in the analog domain while performing the required adjustments (such as channel select filtering) in a digital base band circuitry.
Traditionally, radio receivers are implemented as a superheterodyne or a direct conversion receiver. Both of these are well known in the art. For example, the direct conversion receiver implemented on an integrated circuit usually comprises a low noise amplifier, a quadrature down conversion mixer and separate analog lowpass filters and A/D-converters for the I- and Q-branch.
In order to embed several receiver functions into a single block, frequency translating band pass delta sigma modulators have been proposed. A delta sigma modulator (also called ΔΣ-modulator, sigma-delta modulator, or ΣΔ-modulator) is an oversampling analog-to-digital converter capable of noise shaping. The basic idea in frequency translating delta sigma modulators is to combine both band pass and lowpass modulator stages so that at least the first modulator stage is a band pass stage while the rest are lowpass modulator stages. Naturally, this means that the signal is quadrature down converted with a subsampler or a frequency mixer at the last (or only) band pass modulator stage output. Similarly, the feedback signal has to be quadrature up converted with a frequency mixer for the input modulator stage.
Known problems in A/D converters are related to linearity and so called 1/f-noise. Frequency translating delta sigma modulators provide a partial solution to these problems. Embedding down conversion mixer in a modulator feedback loop reduces the nonlinearities of the mixer and, furthermore, a band pass modulator input stage will reduce the effects of 1/f-noise for the whole A/D-converter since input signal frequency is well above 1/f corner frequency. Similarly, in a fully continuous-time modulator, sinusoidal waveforms in feedback path frequency up converter can be used to reduce timing jitter provided that sinusoidal LO waveform zero crossings are synchronized with a D/A-converter latching instant.
However, the performance of known frequency translating delta sigma modulators is far from the requirements of radio receivers of modern telecommunication systems. There are several reasons for this.
First, most of the presented band pass modulators utilize low-Q resonators in the first stage. Thus, they are less frequency-selective than normal lowpass delta sigma modulators. This results in a band pass channel filter of higher stopband attenuation requirements than those of a lowpass channel filter in a direct conversion receiver.
Second, an unfiltered 1-bit bitstream feedback signal contains high spectral energy at frequencies that may become mixed with the desired signal band in up conversion. Therefore, resolution in prior art modulators is most likely limited by nonidealities in feedback D/A-conversion and/or in the frequency up conversion. All nonlinearities in the feedback path will further degrade the modulator performance.